With digitalization of information society, a writable medium is required to record/reproduce information at high density. In order to improve a recording capacity, various mediums have been arranged as a so-called writable optical information recording medium.
Furthermore, in order to realize high density recording/reproducing, for example, there have been performed the following trials: (i) a wavelength of a recording/reproducing laser beam is made shorter; (ii) a numerical aperture (NA) of an objective lens for converging a laser beam onto an optical information recording medium is increased; (iii) a recording layer of the optical information recording medium is multi-layered; (iv) a mask layer (referred to also as an optical shutter layer) for partially shielding light having an original spot size is formed on the optical information recording medium so as to substantially reduce an optical spot diameter of the laser beam; and the like.
The method in which the mask layer is formed on the optical information recording medium so as to substantially reduce the spot diameter is disclosed, for example, in Patent Document 1 (Japanese Unexamined Patent Publication No. 12673/1993 (Tokukaihei 5-12673)(Publication date: Jan. 22, 1993)) and Patent Document 2 (Japanese Unexamined Patent Publication No. 12715/1993 (Tokukaihei 5-12715)(Publication date: Jan. 22, 1993): the corresponding U.S. Pat. No. 5,246,758(A1)).
Specifically, Patent Document 1 discloses an optical disc device which records/reproduces information in accordance with reflectance variation caused by bumpy pits formed on a substrate of an optical disc, wherein a temperature-dependent optical shutter layer is formed. The temperature-dependent optical shutter layer is arranged so that: an upper portion (a disc surface to which a laser beam is irradiated from the objective lens) of an information recording/reproducing layer which records a signal absorbs a reproduction beam having a certain wavelength at a normal temperature, and stops absorbing the reproduction beam having the foregoing wavelength due to temperature rise caused by the reproduction beam power, and absorbs the reproduction beam having the foregoing wavelength again due to temperature drop after the reproduction beam passes through the reproduction light-converged spot.
Further, Patent Document 1 discloses an optical disc device which records/reproduces information in accordance with reflectance variation caused by a phase change of a recording material, wherein a temperature-dependent optical shutter layer is formed. The temperature-dependent optical shutter is arranged so that: an upper portion (a disc surface to which a laser beam is irradiated from the objective lens) of an information recording/reproducing layer which records a signal absorbs a reproduction beam having a certain wavelength at a normal temperature, and stops absorbing the reproduction beam having the foregoing wavelength due to temperature rise caused by the reproduction beam power, and absorbs the reproduction beam having the foregoing wavelength again due to temperature drop after the reproduction beam passes through the reproduction light-converged spot.
Patent Document 2 discloses an optical recording medium having a recording film provided on a substrate, wherein an optical shutter is provided on the recording film so as to be positioned before a point receiving a reading light beam or a recording light beam. The optical shutter layer includes as a main component a thermochromism material which is in a coloring state which prevents transmission of the reading light beam before irradiation of the reading light beam and partially uncolors a central portion of a reading light beam receiving portion (light transmittance state) with temperature rise of the reading light beam receiving portion.
According to these techniques, in the optical information recording medium having bumpy pits or a recording film provided on a substrate, the mask layer is provided on the pits or the recording film so as to be positioned in the light receiving side. Generally, the mask layer is made of thermochromic material or phase change material. The central portion of the light receiving portion of the mask layer receives a large quantity of light such as the reading light beam, so that temperature of the central portion rises due to the light irradiation. Thus, the central portion of the light receiving portion of the mask layer optically or thermally changes, and partially uncolors, so as to be in a light transmittance state. Meanwhile, in a less-light receiving portion positioned in a periphery of the light receiving portion or in a portion receiving no light, temperature less rises due to irradiation of less light or temperature does not rise at all, so that optical transmissivity becomes small. Thus, light is shielded.
As a result, a substantial spot size corresponds to a size of the central portion of the light receiving portion, so that it is possible to reduce the substantial spot size. That is, in the mask layer, light is allowed to pass through only a portion whose optical intensity distribution is high, so that a spot diameter of the incident light is substantially reduced, thereby recording/reproducing information with a smaller pit. As a result, it is possible to realize high density recording/reproducing with respect to the optical information recording medium.
Patent Document 3 (Japanese Unexamined Patent Publication No. 14093/2004 (Tokukai 2004-14093)(Publication date: Jan. 15, 2004): the corresponding US Publication No. U.S. 2004/0032822 A1) discloses a playback-only optical disc which includes a zinc oxide thin film using near-field light.
The zinc oxide layer is positioned in the backside of the reflection layer in view of the light receiving side. Further, the near-field light is used, so that the zinc oxide layer is comparatively thin (5 nm to 100 nm). Thus, it is necessary to provide a transparent dielectric material film between the reflection layer and the zinc oxide layer so that these layers are not positioned adjacent to each other.
Patent Document 4 (Japanese Unexamined Patent Publication No. 320857/1998 (Tokukaihei 10-320857)(Publication date: Dec. 4, 1998): the corresponding U.S. Pat. No. 6,187,406 B1, U.S. Pat. No. 6,606,291 B2, U.S. Pat. No. 6,744,717 B2) discloses an arrangement in which: zinc oxide is included as a material used in an optical recording medium for super-resolution reproduction, and a band gap is utilized.
However, it is disclosed that: this technique utilizes a Stark effect (electronic excitation and absorption saturation) of a semiconductor so as to obtain a super-resolution effect, and it is most preferable that a wavelength corresponding to a band gap is larger than a reproduction wavelength. Further, increase in the transmissivity in a photon mode is utilized.
Each of the mask layers (optical shutter layers) recited in Patent Document 1 and Patent Document 2 is made of thermochromic material or phase change material. The foregoing material exhibits a mask effect by melting when temperature rises to a certain level. A material in a melting state has higher fluidity, so that a composition and a shape thereof in an initial state are likely to change. Thus, in the optical information recording medium having the mask layer which exhibits the mask effect when temperature rises to a certain level, in case where information is repeatedly recorded and reproduced, the mask effect gradually drops due to deviation in the composition and the shape of the mask layer. Thus, the foregoing optical information recording medium raises such a problem that the mask effect becomes substantially lost due to several thousands repetitions of the recording/reproducing operation. Thus, durability of the conventional optical information recording medium is insufficient.
Further, Patent Document 1 has such recitation that “the temperature-dependent optical transmissivity variable medium is made of a polymer material or an organic material for example, . . . and the optical transmissivity of the material becomes higher in a high temperature region for example. The transmissivity variation may be such that melting of the material raises the optical transmissivity, or the transmissivity variation may be based on variation in regularity of a molecular arrangement like a liquid crystal material. Further, it may be so arranged that: like the phase change material, crystallization caused by heating and cooling substances such as chalcogenide adhering in an amorphous state results in variation of the optical transmissivity.”
However, this recitation fails to specifically recite an example of a material whose optical transmissivity becomes high in a high temperature range. Thus, the invention of Patent Document 1 cannot be realized by person with ordinary skill in the art in accordance with a technique recited in Patent Document 1 and common techniques at the time of publication of Patent Document 1.
The optical disc recited in Patent Document 3 raises such a problem that: the near-field light is utilized, so that relatively large laser power is required, which results in a low ratio of light-signal conversion.
Further, the optical recording medium recited in Patent Document 4 has a low transmissivity (theoretically 0) with respect to a reproduction optical wavelength in an unexcited state, so that this arrangement is not advantageous for a transmissive multi-layered structure. Further, the optical recording medium recited in Patent Document 4 raises such a problem that: it is necessary to make the excitation wavelength and the reproduction wavelength correspond to each other and it is difficult to control this operation.